There has been a continuing problem associated with the spray application of solvent-thinned liquid coating compositions due to wide fluctuations in the ambient atmosphere in the spray booth. Liquid coating compositions adapted for spray application are normally thinned with solvent to reduce their viscosity so as to provide for optimum fragmentation or atomization, achieving uniform coverage of the surface of the substrate being coated. The solvent-thinned liquid coating composition suitable for spray application generally has a viscosity which is insufficient to prevent objectionable sagging or running of the liquid film when applied at reasonable thicknesses to vertical surfaces. This problem is overcome by a controlled volatilization of solvent from the liquid droplets in the spray during the course of their travel from the nozzle to the surface of the substrate. The desired degree of vaporization of solvent can be controlled to some extent by a careful blend of organic solvents and by adjusting the distance between the nozzle and the surface being coated.
While the adjustment in the types of solvents employed in organic solvent-thinned coating compositions has overcome problems associated with wide temperature fluctations in the spray booth environment in the past, governmental restrictions on the flash point of such organic solvent paint systems has occasioned problems in achieving satisfactory drying of the spray pattern employing conventional paint spraying equipment. This problem has become particularly pronounced when employing conventional spray equipment for applying water-thinned liquid coating compositions in which the temperature as well as the humidity of the ambient atmosphere in the spray booth materially affect the volatilization of the water from the spray and wherein the water itself is of relatively low volatility in comparison to conventional organic solvents employed for formulating organic solvent-thinned paint systems. During periods of relatively high humidity, considerable difficulty is encountered in applying water-thinned coating compositions in the form of a liquid film on vertical surfaces without incurring an objectionable running or sagging of the liquid film down the painted surface. At extremely high humidity levels, it is almost impossible to satisfactorily spray such aqueous paints due to the minimal vaporization of water from the spray in route to the surface. Attempts to increase the rate of vaporization of water from such aqueous paint systems by utilizing higher pressure atomizing air and positioning the spray gun or nozzle further from the surface to be coated has been found unsatisfactory in many instances and has also been costly due to the loss or carry off of the fine liquid mist particles in the air passing through the spray booth as a result of "overspray".
In recognition of this problem with both organic solvent and aqueous solvent-thinned liquid coating compositions, various techniques have heretofore been proposed including the use of heated pressurized air for effecting an atomization of the coating composition, heating the liquid coating composition itself prior to fragmentation, as well as supplying heated air such as disclosed in U.S. Pat. No. 2,980,786 into the spray pattern at a position forwardly of the nozzle. Neither of the foregoing techniques have been satisfactory from a commercial standpoint in solving the problems associated with the spray application of solvent-thinned coating compositions, and particularly, aqueous paint systems which are being more widely used to reduce organic solvent emissions.
The present invention provides an apparatus and a method for the spray application of solvent-thinned coating compositions, and particularly aqueous paint systems, whereby a controlled degree of vaporization or drying of the liquid droplets in the spray is effected achieving uniform coverage of a substrate with a liquid paint film having a smooth surface and without any objectionable sagging or running of the liquid film in spite of its application in appreciable thicknesses of up to about 2 mils on a dry-film basis.